Conventionally, an image forming apparatus for outputting an image signal as a visible image on a recording medium, such as a sheet, adopts a method, generally called as xerography. Such an image forming apparatus forms an electrostatic pattern by an optical writing means on a developer having electrical-optical properties, namely, on a photoreceptor, and allows toner which is developing particles to adhere to the electrostatic pattern so as to develop it. Thereafter, the image forming apparatus transfers an developed image to a recording medium such as a sheet so as to form an image signal as a visible image on a recording medium. Concretely, the image signal is converted into a light signal by a light generating unit, such as a laser, an LED (Light Emitting Diode), and the light is irradiated to the photoreceptor which has been uniformly charged so as to form the electrostatic pattern according to light intensity on the surface of the photoreceptor. Successively, the charged toner is allowed to contact with or to fly to the electrostatic pattern so that developing is made, and a toner image is formed on the surface of the photoreceptor. Then, after the toner image is transferred to the recording medium by electrical attractive force, pressure or both of them, the toner image on the recording medium is fixed thereon by pressure, heat or both of them.
In addition, another image forming apparatus forms a charge pattern according to an image signal on a dielectric drum and develops the charge pattern so as to obtain a visible image on a recording medium. This image forming apparatus includes a charged particle current generator, a charged particle current control grid and an dielectric drum as a developer. When a voltage to be applied to the charged particle current control grid is controlled according to an image signal, the charged particle current to be generated by the charged particle current generator is controlled. Then, a charge pattern according to the image signal is formed on the dielectric drum by the charged particles, and the charge pattern is developed by toner so that a toner image is formed on the dielectric drum. Thereafter, in the same manner as that of the previously-mentioned image forming apparatus, the toner image is transferred to and fixed on the recording medium.
However, In such a kind of the image forming apparatus, after the image signal is once formed as an electrostatic latent image on the developer, such as a photoreceptor or a dielectric drum, a toner image is obtained by developing the electrostatic latent image by using toner. Therefore, in the above image forming apparatus, a developer with a special structure for forming the electrostatic latent image is required, and also electrostatic latent image writing means and charge eliminating means for eliminating residual charges of the developer are required. Moreover, a structure that transfers the toner image formed on the developer to the recording medium is complicated. For this reason, an arrangement of the image forming apparatus becomes complicated and there is a limit in miniaturizing the apparatus.
Meanwhile, for example, PCT Unexamined Patent Publication No. 1-503221/1989 (Tokuhyohei 1-503221) discloses an image forming apparatus for directly forming a toner image on a recording medium such as a sheet without the process mentioned above for forming the toner image once on the developer. In this image forming apparatus, an electric field, which allows the toner to fly from a toner holder towards a plate electrode, is given between the toner holder and the plate electrode, and the flying of the toner is controlled by a mesh-like grid electrode provided between the toner holder and the plate electrode. Thereafter, the toner image is directly formed on the recording medium provided on a side opposite to the toner holder on the plate electrode.
However, in the image forming apparatus disclosed in the above Publication, since the grid electrode has a mesh-like shape, in order to obtain an image with excellent quality by satisfactorily controlling the flying of toner, an arrangement of the grid electrode becomes complicated. For this reason, it is hard to mass-produce the grid electrodes, thereby decreasing mass productivity of the image forming apparatus.
Therefore, in order to obtain a grid electrode with a simple arrangement, the inventor of the present invention discovered a grid electrode arranged such that an electrode layer is formed on an insulating substrate made of resin after due examination. The grid electrode (control electrode) is obtained such that after an electrode layer made of a metal film is formed on one surface of a resin film, which is provided to a generally flexible substrate, etc., made of polyimide, polyester, polyethylene, etc., by using a prescribed method, a protective layer made of polyimide, polyester, polyethylene, etc. is formed so as to cover the electrode layer, and a plurality of holes (gates) which are passage sections for toner (developing particles) are provided to the resin film and the protective layer by using a prescribed method. Therefore, the arrangement of the grid electrode becomes simple, and the grid electrode can be easily mass-produced. When the grid electrode is used, the flying of the toner is satisfactorily controlled so that an image with excellent quality can be obtained.
However, in the above grid electrode, when the toner which is charged particles flies and contacts with the resin film or the protective layer, attractive force is generated between a surface of the resin film or of the protective layer and the toner due to electrostatic force, and then the toner adheres to the surface of the resin film or of the protective layer. In other words, the toner is held on the surface of the resin film or of the protective layer by the electrostatic force. For this reason, when the grid electrode is used for a long time or frequently, a lot of toner adheres to the circumference of the holes, thereby blocking the holes. Therefore, in order to obtain excellent image quality stably for a long time, the grid electrode should be maintained, namely, cleaned or replaced so that the toner which has adhered to the holes are removed, or the grid electrode should be replaced according to frequency of usage.